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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-71018-9
Cover image: Pixabay.Com
Cover design by Russell Richardson
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Madhuri Hembram,
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Preface
Soil chemistry refers to the chemical reactions in soils that affect the growth and nutrition of plants. Applied soil chemistry is an interdisciplinary field covering soil, water, plants and atmosphere, which impacts plant, animal and human health. Water and nutrients are provided in different types of soil that are home to microorganisms and many other creatures and plants. The properties of these soils affect the crop production of agricultural fields; therefore, this discipline provides support to the sustainable agricultural management of soils.
State-of-the-art information regarding applied soil sciences is explored in this book. In addition to the fundamentals of soil chemistry, model concepts, principles, chemical reactions, functions, chemical recycling, chemical weathering, acid-base chemistry, carbon sequestration, and nutrient availability of soils are highlighted. Also included among the topics are the chemistry of heavy metals in soil environments, ion-exchange processes on clay, along with relevant analytical tools and applications. This book will help the reader understand soil characteristics by targeting soil chemical reactions and interactions and their applications. Since the chapters were written by noted professionals in the field, it will be an excellent reference guide for students, faculty, researchers and professionals in the field of environmental science, earth science, soil chemistry, and agroecology. The subject matter covered in each chapter is summarized below.
Chapter 1 provides details on the significance of soils as a carbon store. Retaining and ideally boosting carbon in soils helps to inhibit its buildup in the atmosphere. It takes some time for soil carbon to become mineralized, whereas soil erosion and tillage continuously release some soil carbon into the atmosphere.
Chapter 2 provides background information on the chemical weathering of minerals. A discussion of the weathering sequence of minerals from the soil mainly sheds light on the factors which control the rate of chemical weathering, including temperature and time factors, biotic process,
oxidation, reduction, comparative stability of various minerals, water, leaching, and acidity.
Chapter 3 discusses the effect that agricultural management systems have on soil health by evaluating reduced processing systems, organic fertilizers, and biological control of weeds, along with their effect on the bioeconomy, including agriculture, forestry, fishing, aquaculture, woodworking, biorefineries, and nanobiotechnology. The data provided is based on the evaluation of indicators of physical, chemical, biological, microbiological, and biochemical soil health. The benefits of soil resilience and better adaptation to extreme events are also included.
Chapter 4 discusses the advances in soil chemistry which are based on the analysis of the soil at an in-situ level and the presence of carbon speciation in soil. The analysis of the soil using sensors, the internet of things, machine learning, artificial intelligence, and drones with big data are also discussed.
Chapter 5 details the various components of soil, including solid, liquid and gaseous phases. Different soil characteristics like structure, soil color, texture, bulk density and particle size are discussed in detail. These are important parameters for understanding soil behavior. Sorption behavior of soil for removal of heavy metals is also discussed.
Chapter 6 relates to inclusive edaphology and environmental reactions. Parameters such as composition, textures, soil organic matter, salinity, acidity and chemical reaction of soil are discussed. The main focus is on different kinds of contaminants like pesticides, modern agriculture, synthetic chemicals and their effects on plant growth.
Chapter 7 describes fertilization and fertilizer types. The purpose of fertilization and its application methods are discussed. Fertilizers can be classified into two types depending on the source from which they are obtained: organic (natural) fertilizer and chemical fertilizer. The importance of the use of fertilizers in agricultural lands for sustainable agriculture is explained.
Chapter 8 discusses several approaches which are used to control the overabundance of heavy metals present in soil. The focus of this chapter are the techniques utilized in the past few years to estimate heavy metal content and its mitigation process.
Chapter 9 details the importance and benefits of modeling studies about the retention and mobilization of pollutants. Recently applied models and primary outcomes of modeling studies are also discussed. Additionally, several personal computer programs, which are used for running models, are exemplified in this study.
Chapter 10 provides details of the different laws of soil chemistry. The reaction of ions with the variable charged mineral surfaces, organic matters and clay minerals are discussed. The role of water movement through saturated and unsaturated media is also discussed, with a major focus on gravitational and capillary forces.
Chapter 11 discusses various aspects of assessing the quality of soil using mechanical and physicochemical parameters. Mechanical parameters, which play an essential role in plant growth, are also discussed. There is also a discussion of ion-exchange properties, which is the primary focus of the studies on physicochemical parameters.
Chapter 12 explains the necessary soil functions that could lead to an increase in agricultural production. Essential soil biological processes and their relationship with soil pH are also discussed along with microbial ecophysiological indicators and activities of soil enzymes.
Chapter 13 provides detailed information on the application of pesticides in agriculture. It also elaborates on the numerous types of pesticides, including neonicotinoids, pyrethroids, organochlorines, organophosphates, triazines, carbamates, and pyrethroids. The merits and demerits that are associated with chemical pesticides are also highlighted. This chapter also includes the modes of action and the detrimental effects of these pesticides.
February 15, 2021
Inamuddin, Mohd Imran Ahamed, Rajender Boddula and Tariq Altalhi
Potential and Challenges of Carbon Sequestration in Soils
Erfan Sadatshojaei1*, David A. Wood2 and Mohammad Reza Rahimpour3
1Department of Chemical Engineering, Shiraz University, Shiraz, Iran
2DWA Energy Limited, Bassingham, Lincoln, United Kingdom
3Department of Chemical Engineering, Shiraz University, Shiraz, Iran
Abstract
Terrestrial soils, by volume, represent the most significant land-based carbon store on our planet. Over time, soils absorb carbon from a wide range of organisms as they respire during life and decompose after their demise. Carbon currently residing in the upper soil layers constitutes more than the combined quantity of carbon in land-surface vegetation and the atmosphere. Retaining and ideally boosting that carbon store in soils and preventing that carbon entering the atmosphere is of paramount importance in the fight against climate change. Almost 50% of global soils within about 1 m of the surface have been disturbed by agriculture releasing at least some of the carbon they store to the atmosphere. Carbon ideally needs to become mineralized in soils if it is to be stabilized and sequestered in the subsurface over the long term. Unfortunately, a significant portion of carbon in soils has a relative rapid turnover time, or low residence time, and is returned to the atmosphere as carbon dioxide via soil respiration processes. Whereas, it takes much longer for some of the soil carbon to be converted to stable mineralized forms. Soil erosion, as well as tillage, plays a significant role in releasing some soil carbon to the atmosphere. Converting significant areas of croplands and grazing lands to forests, grassland, and wetlands is the best option currently available for increasing the soils uptake of carbon from the atmosphere. Additionally, plant large quantities of perennial deep-rooted, fast growing bioenergy crops, such as switchgrass and miscanthus, can increment the carbon storage potential of grassland soils. The aggressive implementation of such actions has the potential to increase global soil carbon storage by between 0.5 and 2.0 Pg C a−1 for several decades. This could
Today, more than ever, the impact of the advancements in a wide range of technologies and artificial intelligence are influencing progresses and development of human life across the world. In this regard, we can point out ultrasonic application [1], carbon dioxide issue [2], medical research [3–8], and new chemical methods [9] are all advancing rapidly with their impacts being felt more widely. “Sequestration” of combines both the capture of carbon dioxide (CO2) and its long-term isolation from the atmosphere and ocean, storing it safely and securely for thousands of years.
Carbon sequestration in soils, to absorb some of the unwanted CO2 in the atmosphere, mainly involves adopting improvements in land management. This means adopting practices, on a large scale, that convert more atmospheric CO2 into carbon stored in soils than current practices achieve. The main potential to improve carbon management techniques applies to cropland and grazing lands [10]. These improved land use and carbon management techniques strive to increase the rate of biomass entering the soils and/or by reducing the rates of turnover of organic carbon already residing in the soils and by increasing the quantity of soil carbon that becomes mineralized. Through carbon sequestration in soils, CO2 is to a degree stabilized in soils on a semi-permanent basis. However, to achieve this, the CO2 needs to be converted into other materials. These chemical changes are
Sequestration in Soils 3
initiated primarily through organic processes. CO2 is involved in a complex cycle which includes a circulation through the soil influenced by a range of micro-biological activity. Through this circulation, CO2 becomes available in soils to be dissolved by percolating rainwater. This leads to the formation of carbonic acid in near surface fluids.
On a global scale, soils store about 1,500 Pg (petagrams, equivalent to 1,500 gigatonnes) of organic carbon to a depth of one meter, increasing to 2,400 Pg to a depth of 2 m [11]. This means that carbon residing in upper soil layers amounts to more than the combined quantity of carbon in land-surface vegetation and the atmosphere. A little less than 50% of soils globally have been or are in use for agriculture, both cropland and grazing land. The soils involved in cropland activity have almost all been disturbed by some form of tillage. Organic matter within soils can vary between about 1% and 10%.
Subsequently, that carbonic acid reacts with basic cations leading to the creation of secondary carbonates in the short-term, on the scale of years, forming mineralization in near-surface rocks, leading to sustained processes that persists over geological timescales. The creation of secondary carbonates comes mainly from sub-surface weathering and diagenesis reactions with silicate minerals containing calcium and magnesium. Such reactions generate free positively charged ions (cations). Many of these free cations go on to combine with CO2 to form carbonate minerals, particularly calcite and dolomite [12]. However, these pervasive carbonate forming diagenetic processes tend to progress too slowly in their natural cycles to be practically exploited for carbon sequestration purposes. Nevertheless, they do involve substantial quantities of CO2, particularly in alkaline and saline soils present in dry and semi-dry zones [13]. Consequently, the inorganic sub-surface carbon cycle cannot be considered as significant or viable for rapid carbon sequestration in the soils typically found in the soils of wet and temperate zones.
On the other hand, organic carbon can cycle through soils, some returning to the atmosphere very rapidly. In the organic dimensions of the carbon cycle, atmospheric carbon dioxide is stabilized through the photosynthesis conducted by plants, algae, and cyanobacteria to form a range of organic compounds. Although the living organisms initially form glucose during photosynthesis, they transform it into diverse organic compounds, such as cellulose, hemicellulose, and lignin mostly; materials that are useful for biological growth and tissue formation. However, other complex organic materials such as protein, lipids, including more intricate compounds used to provide various benefit to plants and bacteria, are also formed. Land plants direct a significant portion of photosynthetic products to their
roots, some of which are released to the soil as soluble carbon compounds; products termed as rhizoexudates [14].
When plants and bacteria die, their organic constituents are dispersed in soils through decomposition by soil micro-organisms. That decomposition releases much of the CO2 they captured during photosynthesis making its way out of the soil to return to the atmosphere (Figure 1.1). This organic matter-soil decomposition cycle contributes CO2 output to overall soil respiration that includes respiration of plant root and flora and fauna that live in the soil. In addition to the contributions of plants, algae, and cyanobacteria to the carbon cycle through soils, there is a substantial sub-cycle that is related to contributions from animals. The animals consume CO2 in the form of food, with animal excrements and corpses returning to the soil and being decomposed along with plant, algae, and cyanobacteria remnants.
1.1.1 Soil Decomposition Processes
Animals that live in the soil vary from clearly visible spineless animals such as woodlice, centipedes, and earthworms, to smaller, microscopicscale animals, the mesofauna, including mites (arthropods), springtails
Organic materials available in the soil
Figure 1.1 Schematic diagram illustrating the biological contributions to the carbon cycle via terrestrial soils.
Potential and Challenges of Carbon Sequestration in Soils 5
(Collembola a hexapod), and enchytraeidae worm-like (microdrile oligochaete) creatures. Some of the smallest animals in the soil, such as nematodes and protozoa, are among the most effective in the soil decomposition processes [15]. Soil organisms of all sizes and types collectively consume plant, animal, and bacterial debris in the soil. They do this by communition, or the reduction of material from one average particle size to a smaller average particle size, using various physical and biochemical techniques. Fungi and bacteria play a key role in breaking down the structural fabrics of plant materials. These groups are able to convert cellulose and lignin into soluble materials applying complex enzymes to do so. Subsequently, the soluble materials produced are absorbed by the organisms and further metabolized.
Initially, dead plant material located above the ground are, for the most part, decomposed above ground on the soil surface. The soil organisms, weather, and industrial-scale anthropogenic mechanical process such as ploughing, play the substantial role in initiating above-ground and near-surface soil decomposition. In some specific cases, for instance, peat formed in bogs and swamps, the dead plant substances stay at the surface of the soil without time to progress through the complete decomposition process. Instead, it becomes rapidly inhumed by other dead plant substances being added from above, isolating it from abundant oxygen supplies.
A consequence, at completion of the soil decomposition processes, is that carbon is ultimately conveyed from the decaying matter into fungi and soil bacteria. This material is known as microbial biomass. Microbes generate and use this biomass to provide their energy requirements and to create new microbial biomass for growth. That carbon used for energy is converted to CO2 and contribute to soil respiration. However, that portion of the carbon transformed into new microbial biomass, ultimately, is itself consumed or decays upon the demise of the micro-organism and contributes to the ongoing cycle of decomposition. Each successive step in the soil decomposition process involves the consumption of dead biomass by soil organisms, mainly fungi and soil bacteria. Thus, specific carbon molecules pass through many cycles of decay and ultimately end up, over time, either re-emitted to the atmosphere (soil respiration) or fixed by carbon mineralization in the subsurface. On a global scale, the amount of carbon mineralized by soil decomposition is approximately equal to the carbon arriving the soil less the amount re-emitted by soil respiration. Soil respiration returns about 60 Pg a−1 (petagram per year; equivalent to 60 gigatonnes) of carbon to the atmosphere, which is about half of the carbon entering the soil [14].
Total carbon accumulating in soils, termed as net primary production (NPP) from organic sources, varies significantly depending on local climatic conditions, vegetation zones, and ecosystems. NPP varies from about 0.5 t C ha−1 a−1 (tonnes of carbon per hectare per year) in deserts to about 4 t C ha−1 a−1 in grasslands to about 10 t C ha−1 a−1 in tropical rainforests [14]. A direct positive relationship also exists between NPP and the magnitude of carbon released by soil respiration [16].
1.1.2 Organic Compounds Present in Soils
Organic organisms and compounds present in soils are diverse, and the provenance of some of the organic molecules, not present in organisms or identifiable fragments of organisms, is often uncertain. The type and quantity of organic material accumulating in soils is influenced by seasonal weather and biological life cycles. Decomposition processes tend to target the simple biochemical molecules initially; amino acids, nucleic acids, proteins, and sugars. Degradation of the more complex structured molecules, cellulose, hemicellulose, pectins, and polymers takes much longer. Lignins, present in the cell walls of wood and tree bark, takes the longest time to be broken down.
Most biomass consists of complex mixtures of the organic materials mentioned all decaying at different rates and forming physical and chemical mixtures. The presence of lignin in such complex mixtures tends to slow down the decomposition of the components that on their own would decay more rapidly. The degradations of mixtures of organic materials containing cutins and tannins can also be retarded in a similar manner.
Microbes constitute up to about 3% of the organic matter in soils. Bacteria and fungi have their own complex chemical makeups that distinguish them from plant and animal derived biomass. Melanins, derived from some fungi, are not degraded easily and are partly responsible for the dark coloration of some soils. Microbial actions tend to remove visual signs of the structures of organisms they decay turning the organic material into a biochemical soup revealing only their microbial provenance. Some non-biological synthesis of chemical bonds does occur in this biochemical soup, in the presence of oxygen and groundwater of varying acidity, contributing to the humification of soils. The humic materials generated to be stable and not susceptible to fragmentation by biologically produced enzymes [17]. During decomposition the carbon quantity of the organic materials in soils increases from about 40% (undecayed plant material) to about 60% (fully humified soil). Humified soil is not just organic material, it involves mixtures with various inorganic minerals from the weathered outcropping rocks on which
it resides, particularly the finer grained clay minerals. The combination of decayed organic material with inorganic minerals, physically as grain coating and chemically as mineralization reactions, aids the structural integrity of soils and often helps to inhibit mechanical processes involved in soil erosion.
1.1.3 Cycle Time of Carbon in Soils
Despite a global balance between carbon inflow from biological sources to a soil and CO2 output to the atmosphere through soil respiration, that process tends not to be a smooth sequential progression when observed locally in specific soils. On the contrary, carbon introduced in one seasonal cycle may take multiple seasonal and/or annual cycles before it is decayed step-by-step and ultimately becomes mineralized. The CO2 soil respiration output at any point in time comes cumulatively from organic material, at various stages of decay, introduced to the soil during multiple seasonal and annual cycles. The turnover time for carbon in soils varies significantly depending on its stage of maturity and can be determined precisely by carbon isotope dating (14C) techniques. For immature soils, rich in recently demised and introduced biological material, the turnover time is no more than 5 years and could be less than one year. For mineralized soil mixed with inorganic minerals, the turnover time is likely to amount to several decades. The most mature humified soils, from which there is very little carbon inflow and outflow, the carbon turnover times can be measured in thousands of years, meaning that they represent almost inert systems from the perspective of carbon flow [18].
It is often useful to establish turnover times for specific soils. In practice, individual soils will contain components and/or layers displaying a wide range of carbon turnover times. The turnover time of a particular soil layer tends to be influenced by the vegetative geographic zone which determines its sustained humidity and temperature and nutrient content. However, soil instability, due to impacts of extreme seasonal climatic swings and/ or severe weather events that can frequently disturb soils, for example, by increasing leaching rates by ground water, do substantially influence carbon turnover times in some cases.
He et al. (2016) [19] presented 14C dating measurements on 157 globally distributed soil profiles sampled to 1-m depth. Their results revealed that most existing Earth system models tended to substantially underestimate the mean age of carbon in all common soil types. Moreover, that underestimate was more than six-fold (430 ± 50 years versus 3,100 ± 1,800 years). This finding is not good news for soils as a potential carbon sequestration
store, implying that models were making two-fold overestimates of soils global carbon sequestration capabilities. The results indicate that carbon stabilization is a slow process in many soils with long turnover times. This makes soils relative slow-to-develop and passive reservoirs, except on geological time scales, meaning that they cannot absorb large quantities of carbon over short periods of time. This limits soils potential to be a major part of a short-term carbon sequestration solution aimed at removing carbon from the atmosphere on a timescale of decades and over the duration of coming century, which is required to rapidly reverse rising carbon levels in the atmosphere.
The UN’s Intergovernmental Panel on Climate Change (van Diemen, 2019) [20], among other bodies (e.g., Halldorsson et al., 2015 [21]), had previously developed its models for carbon sequestration by soils assuming substantially shorter carbon cycling times. This erroneous assumption led the IPCC to suggest if deforestation on a global scale could be halted about 40 ppm of CO2, about 10% of current levels could be sequestered into soils from the atmosphere. It was believed that combined with major global changes in agricultural practices even more carbon could be absorbed by soils. There is now a more realistic recognition that the carbon absorption ability soils and their turnover periods for carbon cannot be substantially increased in the short-term from the prevailing slow rates [19].
1.2 Influences Impacting Carbon Stabilization Rates in Soils
The amount of organic material present in soils is influenced by many factors (Figure 1.2). Establishing knowledge and understanding of the key influencing factors are essential in determining how carbon uptake and retention of certain soil types can be improved from a long-term carbon capture and sequestration perspective. In a specific soil, the alteration rate of the unit quantity of organic material it contains is established by subtracting the degradation rate from the input rate of biologically derived material. The organic matter degradation rate is positively correlated with the quantity of organic matter present in the soil. This relationship means that progressively the degradation rate converges to the input rate as a soil matures to establish an environmentally adjusting equilibrium with the organic matter remaining constant from that point. The quantity of carbon resident and fixed in a unit mass of soil can therefore be enhanced by either reducing the rate of its degradation of organic matter or accelerating the input of organic matter. On the other hand, more carbon can also be fixed
Key factors influencing carbon quantities and characteristics in soils Additional interactions and feedbacks between these factors
Figure 1.2 Schematic diagram showing how the key variables of climate, vegetation, and soil characteristics impact organic matter concentrations in soils (modified after Sun et al., 2019 [22], who developed the diagram with a specific focus on forest ecosystems).
in a soil system if more mature humified material is continuously produced. This is because the humified material tends to be inert with carbon contributing to its formation, but unlikely to leave it for millennia until it is eroded by geological processes or excavated by anthropogenic activities.
1.2.1 Weather Conditions and Fluctuations
Prevailing weather conditions substantially influence the quantity of carbon present in soils. This is not simply due to differences in specific climatic zones and geographical environments that clearly plays a substantial role. It is well established that the carbon content of soils reduces from the equator to the poles, mainly as a consequence of climatic variations [23]. Specifically, it is biophysical variables such as humidity and temperature conditions in a soil that determine the rates of microbial and bacterial
metabolism. Up to a point higher temperatures and humidity tend to increase metabolic rates; whereas, water-saturated soils, starved of oxygen result in a decrease in metabolic rates. It is the latter conditions that are responsible for massive deposits of peat, formed in cooler and wetter regions and tropical regions, resulting in massive carbon sinks developed over millennia. In many environments, soil water content is the dominant factor in determining the quantity of carbon preserved in peats. In cool or warm arid environments, the lack of soil moisture tends to inhibit the degradation of organic matter but carbon input tends to be low in such soils, leading to their low NPP.
Of course, fluctuating weather conditions also influence soil erosion rates in specific areas. Even if soil temperature and humidity are well balanced to fix large quantities of carbon, that is not much use as carbon sequestration sink if large quantities of the soils produced are frequently eroded by deluges or floods related to periodic or seasonal extreme weather events.
As soils tend to hold less carbon at higher temperatures, Sun et al. (2019) [22] considered the potential release of carbon to the atmosphere from forest soils based on a range of global warming scenarios. They estimated that by the end of this century, the top 20 cm of forest soils globally would release some 6.58 Pg C to the atmosphere if there was just a 1°C increase in global mean air temperature. On the other hand, they suggested that those soils could release as much as 26.3 Pg C to the atmosphere if there was a 4°C increase in global mean air temperature. In either case, the addition of carbon to the atmosphere would likely exceed carbon uptake from reforestation and forest growth.
Rate of decomposition and availability of carbon in soil to microorganisms is often used to distinguish two type of organic carbon in soil: labile and recalcitrant [24]. Labile soil carbon tends to be associated with microbial biomass, dissolved organic matter that is easily oxidized and broken down. Recalcitrant organic carbon in the soil are compound that are resistant to microbial decomposition. Recalcitrant organic carbon tends to be associated with soil mineral particles. Zhang and Zhou (2018) [25] estimate that more than 80% of mineralized soil carbon is derived from the recalcitrant pool of soil organic matter in the temperate forests of northern China. In those forests, the quantity of mineralized soil organic carbon slightly increased with soil moisture content. Mineralization of soil carbon clearly plays a key role in the carbon cycle in terms of determining soil’s ability to store quantities of carbon in a stable manner over long periods of time. That ability is strongly influenced by temperature and moisture content.
1.2.2 Plant and Natural Biomass Inputs
The quantity of carbon in a soil is positively correlated with its input rate of biologically derived material. Increasing that input rate is therefore likely to increase a soil carbon stock. In agricultural regions, this can be achieved, to an extent, by minimizing the time the land remains fallow without vegetation, although in arid regions fallow periods are often essential to restrict overuse of limited water resources. Introducing higher-yield, fast-growing crops and grass leys into crop rotation sequences tends to increase the amount of organic material entering the soil due to the higher biomass production above ground [26]. For instance, a crop rotation including deep-rooted grasses enhanced the organic content in soils from savannah environments by as much as 70 t C ha−1 [27]. Ensuring that maximum quantities of the residual wastes from crop harvests are allowed back into the soil in agricultural regions enhances soil organic content [28]. Increasing biomass production with the aid of nitrogen-rich fertilizers tends to work well in temperate climates but not so well in tropical climates [29]. However, excess nitrogen in rivers and in the oceans surrounding major river deltas is known to have a detrimental effect on biodiversity.
1.2.3 Organic Enrichment Treatments
Introducing organic supplements to agricultural land that is over and above the carbon input resulting from the rotation cycle of crops grown on the land does act to increase the resulting carbon stored in a soil. The simplest way to achieve this is to extensively mulch land with composts and manures. In some developing countries and communities, manure/dung is dried and combusted as low-grade domestic and communal fuel, rather returned to the land. Composted municipal waste and sewage sludges are also applied as organic supplements to farmland at agricultural scales. These can introduce harmful chemicals, such as heavy metals, into soils, so their compositions have to be carefully monitored. Such treatments can also have negative impacts on the biodiversity of both flora and fauna supported by the land if applied inappropriately. Arable farmland generally responds well to such supplements, but grasslands growing on poor soils can be damaged by them [30, 31].
1.2.4 Tilled and Ploughed Agricultural Land
Whereas tilling prepares land for cultivation by mixing particles as a till is dragged through a soil ploughing tends to fragment clumps of soil and
bury weeds and crop residues by penetrating deeper into the soil layers. Both activities are generally referred to as tillage. These activities typically result in higher carbon losses from cultivated soils than undisturbed grassland or forest soils, which typically possess the lower levels of carbon in their undisturbed condition. Deforestation of land to create new land for cultivation often leads to their soils being rapidly degraded in carbon and certain mineral components by groundwater leaching and enhanced microbial metabolism that increases the rate of soil respiration. Increased soil oxygen levels and changes in their temperature and humidity by tillage disruption are typically the causes of enhanced carbon losses from soils, both in the form of CO2 to the atmosphere and as leached organic compounds deeper into the subsurface [32]. Replacing ploughing with less disruptive tilling using shallow-tine equipment can reduce carbon losses from a soil substantially. Whereas ploughing can result in greater than 25% carbon loss, replacing it with shallow tilling can result in just 5% or 6% carbon loss [33]. Soil carbon content can be reduced rapidly the introduction of arable cultivation techniques involving some level of tillage. Buhre et al. (2005) [34] recorded carbon losses of up to 11% from soils subjected to just one cultivation cycle.
1.2.5 Pasture Managed for Livestock Grazing
Grasslands are typically able to sustain higher soil carbon contents than soils of arable land not treated with organic supplements. Moreover, best practices in grazing land management can improve the organic density of grass sward, that is, the surface soil layer constituted by a mass of grasses and their roots. This, in turn, may increase soil carbon levels by greater than 1.0 t C ha−1 a−1 [13, 28]. However, poor grassland management, which tends to be the norm in deforested areas, associated with over-grazing with livestock exacerbates carbon loss, soil erosion, and reductions in biodiversity at the macro and micro levels.
1.2.6 Irrigated Arable Lands and Their Associated Drainage
Water movements through soils and water retention by soils have complex interactions with soil carbon levels. Whereas, decomposition rate of organic matter in soils tends to be reduced by waterlogging, the introduction of artificial drainage, for example, draining peat bogs and fens, often rapidly degrades their carbon contents and microbial activity. Such actions led to the thickness of peat in eastern England reducing by some 4 m due to shrinkage due to water reduction in the peat and by reduced carbon
content with more CO2 lost to the atmosphere [35]. Remedial actions involving artificially increasing water table levels is able can arrest or slow down carbon losses from artificially drained soils [36]. On the other hand, for the most part, projects to artificially irrigate land have positive outcomes for soil carbon contents. They achieve this by raising crop yields and increasing organic matter from crop residues and/or organic supplements making its way into the soil [28, 37].
1.2.7 Uncertain Impacts of Soil Erosion and Redistribution on Its Carbon Store
Soils are not static systems. Climatic and geological processes act continuously to impact soils. Several of these processes disturb and redistribute soils due to erosion and transportation actions associated with the forces exerted by blowing winds and/or flowing water. Soil erosion ultimately moves the organic content of soil from its place of original formation to another, or ultimately distribute portions of soil from one location to several subsequent locations. Similar to the anthropogenic consequences of tillage, natural erosion processes lead to considerable soil disruption that is likely to increase the degradation rate of its organic matter [38]. However, if soils are merely displaced by a single erosion event, for example, by a landslip, relatively small amounts of carbon loss are likely to result. Unfortunately, the continuous and repeated nature of erosive forces and events leads soil components to gradually make their way, via fluvial actions, into ocean sediments. Although, the carbon that does ultimately arrive in ocean sediments may then be buried for many millennia and isolated from the ocean and atmosphere [38], much of the carbon temporarily fixed by the original soil formation processes is lost or partially degraded along the way. This makes it almost impossible to quantify on a global scale what quantities of carbon stored in soils is ultimately returned to the atmosphere by the geological forces of erosion. In the short term also, the quantities of crop residues entering a soil system is reduced by the actions of erosion at the surface, typically resulting from extreme or seasonal weatherrelated disturbances.
1.2.8 Fire Impacts on Soil Characteristics
Across the world, fire is relatively frequent but intermittent natural phenomenon impacting large tracts of land. In addition to natural causes of land fires (lightning strikes, volcanic activity, etc.) there are anthropogenic causes such as crop residue burning, forest clearance, accidents, and arson.
Isolated, high-latitude wetlands are much less likely to experience natural land fires caused by lightning strikes [39] than hotter drier regions close to human population centers, where substantial quantities of dry biomass at the land surface is more easily ignited.
Seasonal burning to contain vegetation is a land management technique used in some areas. Although this removes biomass and temporarily reduces carbon input to the underlying soils [40], this practice can ultimately stimulate new, more rapid vegetation growth that will, for a time, enhance carbon input to the soil. The addition of some “charcoal” or charred carbon-rich material to the soil following episodes of burning also contributes to the inert carbon matter in the soil. This component, and artificially produced biochar additions, acts to stabilize some of the carbon within fire-affected or biochar-treated soils. Whether induced naturally or for land management purposes, there is a risk that land fires can ultimately ignite the underlying soils and peats. If this occurs, it releases huge quantities of CO2 to the atmosphere, and it reduces or removes the carbon store in the soils that have taken millennia to accumulate. Such outcomes have been observed associated with forest clearances in Brazil and Indonesia and are likely intermittent natural events over geological time scales.
1.3 Carbon-Sequestration Potential of Specific Vegetation Zones and Ecosystems
Fundamentally, each type of land-surface vegetation type has characteristic carbon quantities and fabrics in its underlying soils. For instance, in temperate climates carbon contents tend to be higher than the global average and increase as latitude increases within the temperate zones. Reductions in temperature and increases in waterlogging of soils at higher temperate latitudes are the main causes of this trend. These changes tend to enable a constant stream of biomass to enter the soils but reduce organic matter degradation. The global average estimated carbon density per unit area of soil in various vegetated zones and ecosystems are listed in Table 1.1.
1.3.1 Croplands
Soils underlying arable croplands typically possess low carbon soil contents (less than about 150 t C ha−1), because they are either under cultivation or
Table 1.1 Estimated soil carbon complements in the top 1 m of common vegetated systems. The ranges provided indicate uncertainty in the global average carbon density for each major vegetated system considered (after Hester et al., 2010) [14].
Ecosystem
Carbon density (t C ha−1)
Wetland 643
Cropland (arable) 80–122
Tundra 127–206
Deserts and semi-deserts 42–57
Temperate grassland and shrubland 99–236
Tropical savanna and grassland 90–117
Boreal forest 247–344
Temperate forest 96–147
Tropical forest 122–123
have been cultivated in the past [14]. Typically, such soils are maintained under the condition that causes them to lose their carbon to the atmosphere and by leaching at a rapid rate. they present a major challenge for sustainable future sequestration of carbon in soils.
1.3.2 Grasslands
Grasslands typically display higher soil carbon contents rather than cropland soils. Table 1.1 displays the carbon density of undisturbed naturally occurring grassland soils. Some crop rotation schemes combine periods of grassland cultivation with periods of arable crop growth. Soils associated with such systems tend to have carbon densities higher than croplands but not as high as natural undisturbed grassland soils. The soil carbon density tends to increase for areas that are laid to grass for higher fractions of the crop rotation cycle. Extensive cultivation of fast-growing bio-energy grasses with deep root systems, such as switchgrass and miscanthus, has the potential to substantially increase the carbon density of grassland and cropland converted to grassland.
